Prior to the inventions described in the aforementioned related patent applications, the liquid spary application of coatings, such as lacquers, enamels and varnishes, was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from painting and finishing operations. For this reason, there has been a great deal of emphasis placed on the development of new coatings technologies which diminish the emission of organic solvent vapors. A number of technologies have emerged as having met most but not all of the performance and application requirements, and at the same time meeting emission requirements and regulations. They are: (a) powder coatings, (b) water-borne dispersions, (c) water-borne solutions, (d) non-aqueous dispersions, and (e) high solids coatings. Each of these technologies has been employed in certain applications and each has found a niche in a particular industry. However, at the present time, none has provided the performance and application properties that were initially expected.
Powder coatings, for example, while providing ultra low emission of organic vapors, are generally characterized as having poor gloss or good gloss with heavy orange peel, poor distinctness of image gloss (DOI), and poor film uniformity. Moreover, to obtain even these limited performance properties generally requires excessive film thickness and/or high curing temperatures. Pigmentation of powder coatings is often difficult, requiring at times milling and extrusion of the polymer-pigment composite mixture followed by cryogenic grinding. In addition, changing colors of the coating often requires its complete cleaning, because of dust contamination of the application equipment and finishing area.
Water-borne coatings, on the other hand, are very difficult to apply under conditions of high relative humidity without serious coating defects. There defects result from the fact that under conditions of high humidity, water evaporates more slowly than the organic cosolvents of the coalescing aid, and as might be expected in the case of aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation. Poor gloss, poor uniformity, and pin holes unfortunately often result. Additionally, water-borne coatings are not as resistant to corrosive environments as are more conventional solvent-borne coatings.
Coatings applied with organic solvents at high solids levels avoid many of the pitfalls of powder and water-borne coatings. However, in these systems, the molecular weight of the polymer has been decreased and reactive functionality has been incorporated therein so that further polymerization and crosslinking can take place after the coating has been applied. It has been hoped that this type of coating will meet the ever-increasing regulatory requirements and yet meet the most exacting coatings performance demands. However, there is a limit as to the ability of this technology to meet the performance requirement of a commercial coating operation. Present high solids systems have difficulty in application to vertical surfaces without running and sagging of the coating. If they possess good reactivity, they often have poor shelf and pot life. However, if they have adequate shelf stability, they cure and/or crosslink slowly or require high temperature to effect an adequate coating on the substrate.
Clearly, what was needed was an environmentally safe, non-polluting diluent that can be used to thin very highly viscous polymer and coatings compositions to liquid spray application consistency. Such a diluent would allow utilization of the best aspects of organic solvent-borne coatings applications and performance while reducing the environmental concerns to an acceptable level. Such a coating system could meet the requirements of shop- and field-applied liquid spray coatings as well as factory-applied finishes and still be in compliance with environmental regulations.
Such a needed diluent was indeed found and is discussed in the aforementioned related applications which teach, among other things, the utilization of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent-borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute these compositions to application viscosity required for liquid spray techniques.
As used herein, it will be understood that a "supercritical fluid" is a material which is at a temperature and pressure such that it is at, above, or slightly below its "critical point". As used herein, the "critical point" is the transition point at which the liquid and gaseous states of a substance merge into each other and represents the combination of the critical temperature and critical pressure for a given substance. The "critical temperature", as used herein, is defined as the temperature above which a gas cannot be liquefied by an increase in pressure. The "critical pressure", as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the critical temperature.
Also as used herein, a "coating material" is meant to include a coating composition containing no supercritical fluid. The coating material may provide a coating on a substrate which is decorative, such as paint; which is an adhesive; which is an insecticide; or the like. The specific nature of the coating material is not critical to the present invention provided that it can be admixed with supercritical fluid and sprayed.
A "coating mixture", as used herein, is meant to include a mixture of a coating material with at least one supercritical fluid.
Aforementioned U.S. Pat. No. 4,923,720 discloses processes and apparatus for the liquid spray application of coatings to a substrate that minimize the use of environmentally undesirable organic diluents. One of the process embodiments of that patent includes:
(1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA0 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon. PA0 (1) means for supplying at least one polymeric compound; PA0 (2) means for supplying at least one active solvent; PA0 (3) means for supplying supercritical carbon dioxide fluid; PA0 (4) means for forming a liquid mixture of components supplied from (1)-(3); and PA0 (5) means for spraying said liquid mixture onto a substrate.
(a) at least one polymeric compound capable of forming a coating on a substrate; PA1 (b) at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture of (a) and (b) to a point suitable for spray application; and
That application is also directed to a liquid spray process in which at least one active organic solvent (c) is admixed with (a) and (b) above prior to the liquid spray application of the resulting mixture to a substrate. The preferred supercritical fluid disclosed is supercritical carbon dioxide. The process employs an apparatus in which the mixture of the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. The apparatus includes:
The apparatus may also provide for (6) means for heating any of said components and/or said liquid mixture of components.
Related copending U.S. patent application Ser. No. 218,910, filed Jul. 14, 1988, is directed to a liquid coatings application process and apparatus in which supercritical fluids, such as supercritical carbon dioxide fluid, are used to reduce to application consistency, viscous coating materials to allow for their application as liquid sprays. The resulting coating mixtures are sprayed by passing the mixture under pressure through an orifice into the environment of the substrate.
Related U.S. patent application Ser. No. 218,896, filed Jul. 14, 1988, is directed to a process and apparatus for coating substrates by a liquid spray in which; (1) supercritical fluid, such as supercritical carbon dioxide fluid, is used as a viscosity reduction diluent for coating materials; (2) the mixture of supercritical fluid and coating material is passed under pressure through an orifice into the environment of the substrate to form the liquid spray; and (3) the liquid spray is electrically charged by a high electrical voltage relative to the substrate.
Related U.S. patent application Ser. No. 327,484, filed Mar. 22, 1989, is directed to coating materials which are particularly suitable for being admixed with at least one supercritical fluid used as a viscosity reduction diluent and then spraying this resultant coating mixture of supercritical fluid and coating material onto a substrate to be coated.
Related U.S. patent application Ser. No. 327,274, filed Mar. 22, 1989, is directed to coating materials containing water and at least one organic solvent which are particularly suitable for being admixed with at least one supercritical fluid used as a viscosity reduction diluent and then spraying this resultant coating mixture of supercritical fluid and coating material onto a substrate to be coated. Processes for spraying this coating mixture are also disclosed.
Related U.S. patent application Ser. No. 326,945, filed Mar. 22, 1989, is directed to adhesive coating materials which optionally contain water, which are particularly suitable for being admixed with at least one supercritical fluid used as a viscosity reduction diluent and then spraying this resultant coating mixture of supercritical fluid and adhesive coating material onto a substrate to be coated. Processes for spraying these adhesive coating mixtures are also disclosed.
Related U.S. patent application Ser. No. 327,273, filed Mar. 22, 1989, is directed to methods and apparatus for spraying liquid compositions by airless spray techniques which avoid fishtail spray patterns and desirably obtain feathered spray patterns.
Related U.S. patent application Ser. No. 327,275, filed Mar. 22, 1989, is directed to methods and apparatus for spraying liquid compositions by airless spray techniques so as to obtain wider spray patterns without having to alter the construction or configuration of conventional nozzles, nozzle tips or spray guns. By means of the invention disclosed therein, the width of a spray pattern may be changed while the spraying operation is being carried out.
Smith, U.S. Pat. No. 4,582,731, patented Apr. 15, 1986, and U.S. Pat. No. 4,734,451, patented Mar. 29, 1988, disclose a method and apparatus for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in organic and supercritical fluid solvents. The concentration of said solutes are described as being quite dilute; on the order of 0.1 percent. In conventional coating applications, the solute concentration is normally 50 times or more greater than this level.
The molecular sprays disclosed in the Smith patents are defined as a spray "of individual molecules (atoms) or very small clusters of solute" which are in the order of about 30 Angstroms in diameter. These "droplets" are more than 10.sup.6 to 10.sup.9 less massive than the droplets formed in conventional application methods that Smith refers to as "liquid spray" applications.
Turning more particularly to the aforementioned related U.S. patent application Ser. No. 218,910, a process is disclosed therein where the coating material and carbon dioxide are pumped from separate pressure reservoirs and proportioned by a variable ratio proportioning pump unit which proportions the two fluids together at a given volume ratio by using two piston pumps slaved together. The correctly proportioned coating material and carbon dioxide are then mixed in a static mixer and pumped on demand into a circulation loop, which circulates the coating mixture at spray pressure and temperature to or through the spray gun(s). The coating mixture is heated in an electric heater to obtain the desired spray temperature and filtered in a fluid filter to remove particulates. The circulation flow in the loop is obtained through the use of a gear pump.
An alternative method of proportioning the coating material and supercritical fluid in a continuous mode is by the use of a mass proportionation apparatus, instead of the volumetric proportionation apparatus discussed above, as described in related U.S. patent application Ser. No. 327,273.
As disclosed in the aforementioned related patent applications, the spray pressure used is a function of the coating material, the supercritical fluid being used, and the viscosity of the coating mixture. The minimum spray pressure is at or slightly below the critical pressure of the supercritical fluid. Generally, the pressure will be below 5000 psi. Preferably, the spray pressure is above the critical pressure of the supercritical fluid and typically is below 3000 psi. If the supercritical fluid is supercritical carbon dioxide fluid, the preferred spray pressure is between 1070 psi and 3000 psi. The most preferred spray pressure is between 1200 psi and 2500 psi.
The spray temperature used is a function of the coating material, the supercritical fluid being used, and the concentration of supercritical fluid in the coating mixture. The minimum spray temperature is generally at or slightly below the critical temperature of the supercritical fluid. The maximum temperature is the highest temperature at which the components of the coating mixture are not significantly thermally degraded during the time that the coating mixture is at that temperature.
If the supercritical fluid is supercritical carbon dioxide fluid, because the supercritical fluid escaping from the spray nozzle could cool to the point of condensing solid carbon dioxide and any ambient water vapor present due to high humidity in the surrounding spray environment, the spray composition is preferably heated prior to atomization. The minimum spray temperature is about 31.degree. C. The maximum temperature is determined by the thermal stability of the components in the coating mixture. The preferred spray temperature is between 35.degree. and 90.degree. C.
Generally, liquid mixtures with greater amounts of supercritical carbon dioxide fluid require higher spray temperatures to counteract the greater cooling effect.
Typically the spray undergoes rapid cooling while it is close to the orifice, so the temperature drops rapidly to near or below ambient temperature. If the spray cools below ambient temperature, entrainment of ambient air into the spray warms the spray to ambient or near ambient temperature before the spray reaches the substrate. This rapid cooling of the spray is beneficial because less active solvent evaporates in the spray in comparison to the amount of solvent lost in conventional heated airless sprays. Therefore, a greater proportion of the solvent is retained in the coating material to aid leveling of the coating on the substrate. Conventional heated airless sprays also cool to ambient temperature before reaching the substrate, because of solvent evaporation and entrainment of ambient air.
The spray temperature may be obtained by heating the coating mixture before it enters the spray gun, by heating the spray gun itself, by circulating the heated coating mixture to or through the spray gun to maintain the spray temperature, or by a combination of such methods. Circulating the heated liquid mixture through the spray gun is preferred to avoid heat loss and to maintain the desired spray temperature.
While the use of such a circulation loop to provide multi-passes of the coating mixture through the spray gun, while spraying or not, is advantageous from the point of view of maintaining the coating mixture at a desirable spray temperature or for the continuous mixing of a coating mixture to prevent settling of undissolved constitutents, such as pigments and the like, such a multi-pass mode may not be desirable for all applications. In particular, when utilizing a multi-pass mode, the coating mixture is subjected to a longer period in which it is heated and indeed, some portions of the coating mixture may be heated indefinitely by such continual circulation. If the coating mixture contains heat sensitive or reactive constituents, such a long residence time is clearly undesirable.
So too, a multi-pass circulation loop also requires the utilization of equipment for providing such circulation, e.g., recirculation pump, circulation loop heater, corresponding piping, and the like. This equipment must all be thoroughly cleaned when changing from one coating mixture to another, particularly when changing colors, for example, thereby increasing the risk of cross-contamination. Moreover, this additional equipment also adds to wasted "dead" volume within the overall apparatus since all of the coating mixture contained in the circulation loop must be removed and discarded when changing to a new coating mixture.
It is apparent, therefore, that it would be desirable to utilize a single-pass mode for supplying coating mixture to the spray gun, at least in some applications, in which there is no circulation provided from the spray gun, passed a heater, a static mixer, a circulation pump, and the like, and then back to the spray gun again, on a continuous basis. Such a single-pass mode is particularly desirable for spray operations that require frequent material changes, such as color changes, or use reactive materials or heat sensitive materials. In particular, as a result of using a single-pass mode: 1) the volume of material to be changed is much smaller, which minimizes the amount of waste material created that must ultimately be disposed of; 2) the time required for color changes or cleaning is much shorter, so that paint line speeds can be higher; and 3) the time the material is heated is much less so that reactive and sensitive materials are much less affected before they are sprayed. Such a single-pass mode is also desirable for limited spraying in such applications as fine finishing, automobile refinishing, touch-up, and the like, where a small amount of coating material is used, particularly when utilizing a portable coating operation. Such single-pass systems are common throughout the industry for coating automobiles, airplanes, appliances, machinery, metal furniture, component parts, and other original equipment manufacturing coating operations; for furniture finishing and refinishing; automotive refinishing and touch-up, especially in the small body repair shops; and in small appliance refinishing and touch-up.
We have found, however, that if the apparatus disclosed in the aforementioned related applications were modified so as to convert the multi-pass mode to a single-pass mode, the resulting spray would generally provide poor atomization which, in turn, would produce a coating on the substrate of poor quality. More specifically, we have found that such poor atomization is directly attributable to spraying the coating mixture at too low a spraying temperature which spraying temperature is decreased by a number of factors heretofore unknown.
Thus, as briefly noted above, the spray temperature is a function of the coating meterial being used, the supercritical fluid being used, and the concentration of such supercritical fluid in the coating mixture. Keeping these variables constant, proper atomization is obtained when the spray temperature is such that fine liquid droplets are obtained generally having an average diameter of one micron or greater. Preferably, these droplets have average diameters of from about 5 to about 1000 microns.
Such proper atomization can easily be observed by the shape and pattern of the spray that is produced signifying that the proper spray temperature is being maintained as the coating mixture is sprayed. In particular, as disclosed in related U.S. patent application Ser. No. 327,273, filed Mar. 22, 1989, a feathered spray pattern is clearly observed when proper atomization of the coating mixture is being obtained in contrast to a typical fishtail pattern. So too, as also disclosed in related U.S. patent application Ser. No. 327,275, filed Mar. 22, 1989, during proper atomization of the coating mixture, it can also be observed that the width of the spray fan is generally much wider than that which would be expected for the particular spray tip being used.
In other words, when a single-pass mode is substituted for a multi-pass mode, we have found that the spray that is produced is not in a feathered spray pattern, and is not wider than that which would be expected, either one of which would indicate that proper atomization is not taking place. Such poor atomization generally indicates that the spray droplets being produced are larger than that desired which, in turn, produces poor quality coatings.
In particular, we have found, when spraying coating mixtures that contain supercritical fluids, such as carbon dioxide, that the spray mixture experiences adverse heat loss when using spray guns with single-pass flow of the coating mixture. We have discovered that the heat loss occurs both from within the gun and from the feeding means which supplies the coating mixture to the spray gun after being heated to the desired temperature. This heat loss, we have discovered, causes the spray temperature to be less than that required to provide proper atomization, as reflected by, for example, the lack of obtaining a feathered spray pattern. Without proper atomization, poor coating quality is obtained.
We have also found that heating the coating mixture to a higher temperature in the heater to compensate for such heat loss is inadequate when the spray is intermittent and not continuous. Steady-state is not obtained and consequently, the spray temperature will fluctuate as the spray is turned on and off. Furthermore, some heat-sensitive coatings cannot tolerate being heated to a higher temperature.
Moreover, at start-up, the heated spray mixture must be purged through the gun to first heat the gun and the feed line, which subjects the spray mixture to an even larger temperature drop than during normal spraying. This purging also wastes coating material and creates a waste disposal problem. It is clear that in the non-circulating, single-pass mode of airless spraying of coating formulations containing supercritical fluids where temperatures must be maintained and controlled to be near or above the critical temperature of the supercritical fluid, which may be above ambient temperature, that presently available commercial spray guns are inadequate and there is a need for improved apparatus and processes which would provide heating of the spray gun and the feed lines by a means other than with the coating mixture itself.
Still further, in contrast to circulating the coating mixture to and from the spray gun as the mixture is being sprayed to provide for continuous mixing and heating, we have found that a single-pass spray gun system has a tendency to allow the settling of non-dissolved components of the coating mixture, such as pigments, metallic flakes and the like. Hence, there is also a need for a means of maintaining the homogeneity of the coating mixture during single-pass operation in addition to the need for maintaining a given temperature level.
Furthermore, when using a single-pass mode or even when using the multi-pass mode disclosed in the aforementioned related applications, we have also noted that, at times, deterioration of the spraying occurs as more and more substrate is sprayed. When spraying first starts, good results are obtained. As spraying continues, however, the spray pattern appears to change getting coarser, which results in the substrate having less than desirable quality. The most notable aberration is "orange peel," which is the formation of circular crater-like formations. Although the film obtained is a continuous one, with the substrate being completely coated, the film is uneven in film thickness, having a dimpled surface. This orange peel condition results in poor gloss and poor distinctness of image. We have discovered that this phenomenon is apparently caused by the cooling of the spray gun nozzle assembly, which includes the spray tip, as spraying is continued over a relatively extended period of time, regardless of whether there is a circulation loop or not.
Particularly, we found that when heated liquid spray mixtures containing supercritical fluid such as carbon dioxide are sprayed through an airless spray nozzle, even with circulation of the liquid mixture through the spray gun, the spray nozzle undergoes cooling, i.e., the spray nozzle temperature drops during spraying.
In conventional heated airless spraying, with circulation of the spray mixture to and through the spray gun, it is expected and observed that after the start of spraying, the spray gun nozzle temperature increases to the temperature inside the gun and substantially maintains that temperature throughout spraying. Since the solvents included in conventional spray mixtures are liquids at abmient conditions and therefore have relatively low vapor pressures, it is not expected by one skilled in the art to observe spray gun nozzle temperature decrease while the coating mixture is under pressure in the spray gun nozzle due to an evaporative cooling phenomenon.
However, when spraying a coating mixture containing a supercritical fluid, as in the present invention, we theorize that nozzle cooling is caused by the supercritical fluid, e.g., carbon dioxide, vaporizing inside the spray nozzle before exiting the orifice. In particular, we believe that the spray mixture undergoes a pressure drop inside the spray gun and, more specifically, inside of the spray nozzle. This pressure drop may be caused by the coating mixture, containing the supercritical fluid, flowing through contractions and expansions such as the valve channel, slots, flow splitters, and chambers in the gun and nozzle downstream of the valve, and turbulence promoters such as diffuser pre-orifices which are typically found in state-of-the-art spray guns. Very large pressure drop occurs as the coating mixture passes through the orifice in the spray tip, which causes rapid cooling as the supercritical fluid rapidly vaporizes from solution. Accordingly, we have discovered that providing a means for heating the spray nozzle would help eliminate these problems.
Still further, when utilizing the apparatus and methods disclosed in the aforementioned related patent applications, we have also noted that when the spraying of a substrate is halted, coating mixture may still "spit" or "ooze" out of the spray nozzle of the spray gun despite the fact that the spray gun has been shut off. This released material may undesirably be entrained into the spray as large droplets when spraying is resumed thereby marring the resulting coating. Alternatively, this spitting of the large droplets may mar the coating directly during flow shut off. Without wishing to be bound by theory, we believe that such "spitting" or "oozing" is being caused by vaporization of the supercritical fluid inside the spray nozzle when the flow valve closes and the coating mixture inside the spray nozzle is quickly depressurized to atmospheric pressure. This is believed caused by the flow of coating mixture which still occurs between the valve located inside of the spray gun and the orifice in the spray tip even when the valve is shut.
Normal or conventional spray materials comprise relatively incompressible liquids and their solvents have relatively low vapor pressures, so little change in volume occurs during depressurization. However, spray solutions that contain carbon dioxide or other supercritical fluids as a solvent are compressible and have a high vapor pressure. Consequently, a large increase in volume occurs during depressurization as the supercritical fluid comes out of solution and expands as a gas. It is this expansion of the coating mixture which we believe causes the material to "spit" or "ooze" out of the spray nozzle. This phenomenon also causes significant cooling of the spray nozzle and the coating mixture left inside it, which causes improper spray atomization when spraying is resumed because proper spray temperature is not maintained. When valving is rapid, this cooling phenomenon can cause noticeable progressive deterioration of the spray and coating such as increased orange peel. The more volume that is present between the inlet valve and the spray tip, the more undesirable evaporative cooling that occurs.
Accordingly, we have also recognized that what is additionally needed is a spray gun design in which there is a reduction of the volume of material remaining downstream of the shut-off valve once spraying has been stopped. Commercially available airless spray gun nozzle assemblies contain enough void volume to cause the said spitting and oozing. Commercially available spray nozzle inserts, such as Spraying Systems No. 15153-NY insert for Airless TC Nozzles, are known to those skilled in the art, but such an insert does not solve the problem, although, it does reduce it somewhat.
Hence, in order to help solve all of the problems that we have now recognized, what is needed is: 1) a means for heating the spray gun and its feed hose, while desirably also providing for in-spray gun mixing; 2) a means of increasing the temperature of the spray nozzle tip; and 3) a means for minimizing the flow volume between the flow valve and the spray orifice of the spray gun. An apparatus which meets these needs would then be able to maintain the proper temperature and homogeneity of the coating mixture required for the proper atomization of the spray mixture, and would also help minimize the material that "spits" of "oozes" out of this cavity when the flow valve is closed. Such an apparatus would thereby help avoid the production of undesirable coatings on the substrate.
None of the problems that we have recognized can be solved using wholly conventionally available spray equipment designed for use with non-compressible fluids.